1. Field of the Invention
The invention relates to an inverse dispersion optical fiber. More particularly, the invention relates to an inverse dispersion optical fiber having reduced optical loss relative to conventional inverse dispersion fiber and that is suitable for compensating dispersion in large effective area positive dispersion fiber.
2. Description of the Related Art
Optical fibers are thin strands of glass or plastic capable of transmitting optical signals, containing relatively large amounts of information, over long distances and with relatively low attenuation. Typically, optical fibers are made by heating and drawing a portion of an optical preform comprising a refractive core region surrounded by a protective cladding region made of glass or other suitable material. Optical fibers drawn from the preform typically are protected further by one or more coatings applied to the cladding region.
Advances in transmission over optical fibers have enabled optical fibers to have enormous bandwidth capabilities. Such bandwidth enables thousands of telephone conversations and hundreds of television channels to be transmitted simultaneously over a hair-thin fiber. Transmission capacity over an optical fiber is increased in wavelength division multiplexing (WDM) systems wherein several channels are multiplexed onto a single fiber, with each channel operating at a different wavelength. However, in WDM systems, nonlinear interactions between channels occurs, such as 4-photon mixing, which severely reduces system capacity. This problem has been largely solved by U.S. Pat. No. 5,327,516 (the ""516 patent), which is owned by the assignee of the present application. The ""516 patent discloses an optical fiber that reduces these nonlinear interactions by introducing a small amount of chromatic dispersion at the operating wavelengths. As the number of WDM channels to be transmitted over a single fiber increases, the optical power carried by the optical fiber also increases. As the optical power increases, the nonlinear effects caused by interaction between the channels also increases. Accordingly, it is desirable for an optical fiber to provide a small amount of chromatic dispersion to each of the WDM channels in order to reduce the nonlinear interactions between the channels, especially in view of ever-increasing bandwidth demands. However, in order to be able to restore the signal after the transmission link, it is important that the dispersion introduced vary as little as possible amongst the different WDM channels.
Important advances have been made in the quality of the material used in making optical fibers. In 1970, an acceptable loss for glass fiber was in the range of 20 dB/km, whereas today losses are generally about 0.25 dB/km. The theoretical minimum loss for glass fiber is about 0.16 dB/km, and it occurs at a wavelength of about 1550 nanometers (nm). Dispersion in a glass fiber causes pulse spreading for pulses that include a range of wavelengths, due to the fact that the speed of light in a glass fiber is a function of the transmission wavelength of the light. Pulse broadening is a function of the fiber dispersion, the fiber length and the spectral width of the light source. Dispersion for individual fibers is generally illustrated using a graph (not shown) having dispersion on the vertical axis (in units of picoseconds (ps) per nanometer (nm), or ps/nm) or ps/nm-km (kilometer) and wavelength on the horizontal axis. There can be both positive and negative dispersion, so the vertical axis may range from, for example, xe2x88x92250 to +25 ps/nm km. The wavelength on the horizontal axis at which the dispersion equals zero corresponds to the highest bandwidth for the fiber. However, this wavelength typically does not coincide with the wavelength at which the fiber transmits light with minimum attenuation.
For example, typical single mode fibers generally transmit best (i.e., with minimum attenuation) at 1550 nm, whereas dispersion for the same fiber would be approximately zero at 1310 nm. Also, the aforementioned theoretical minimum loss for glass fiber occurs at the transmission wavelength of about 1550 nm. Because minimum attenuation is prioritized over zero dispersion, the wavelength normally used to transmit over such fibers is typically 1550 nm. Also, Erbium-doped amplifiers, which currently are the most commonly used optical amplifiers for amplifying optical signals carried on a fiber, operate in 1530 to 1565 nm range. Because dispersion for such a fiber normally will be closest to zero at a wavelength of 1310 nm rather than at the optimum transmission wavelength of 1550 nm, attempts are constantly being made to improve dispersion compensation over the transmission path in order to provide best overall system performance (i.e., low optical loss and low dispersion).
In order to improve dispersion compensation at the transmission wavelength of 1550 nm, it is known to couple the transmission fiber, which normally is a positive dispersion fiber (PDF), with an inverse dispersion fiber (IDF). The positive dispersion transmission fiber typically comprises a single mode fiber designed to introduce dispersion in order to reduce the nonlinear interactions between channels. The inverse dispersion fiber has a negative dispersion and negative dispersion slope that match the dispersion characteristics of the positive dispersion transmission fiber (but are opposite in sign) in order to compensate dispersion in a broad wavelength range and minimize the residual dispersion (i.e., dispersion on wavelength channels other than the center wavelength channel being compensated).
A transmission PDF is coupled to a length of IDF by splicing. The combination of the PDF and the IDF has both an intrinsic fiber loss and a splicing loss. Of course, overall optical loss for a transmission link should be kept at a minimum. This is especially true over long transmission links because more amplifiers are needed in order to prevent transmission quality degeneration when the transmission link has larger losses. For example, in trans-oceanic communications systems it is advantageous to use a combination of large effective area PDF and an IDF having matching dispersion and dispersion slope characteristics that are of opposite sign to those of the PDF. This combination results in the minimal accumulation of residual dispersion over the transmission wavelength range. Conventional IDF has a median loss of, for example, approximately, 0.246 db/km at 1550 nm. One way to decrease the overall loss of the transmission link would be to utilize an IDF that has a lower fiber loss than the conventional IDF that is currently being used.
Many features of a fiber, such as an IDF, can be ascertained from the refractive index profile of the fiber. The refractive index profile shows how the index of refraction of the fiber varies as a function of distance from its central axis. Parameters used for describing the refractive index profile generally are referenced to the index of refraction of the outermost layer of glass. Idealized models of refractive-index profile typically comprise axially symmetric rings or regions of different refractive index. However, changing the number, size and/or shape of any one of these regions generally impacts more than one characteristic of the fiber (e.g., dispersion slope is reduced, but bending loss is increased or effective area is decreased). Thus, it is a significant design effort to create a refractive index profile that provides most if not all of the desired features for the fiber, and yet still be readily manufacturable.
It would be desirable to have an IDF with a refractive index profile that provides the IDF with a lower fiber loss than the fiber loss of the conventional IDF currently being used and that provides minimal accumulation of residual dispersion over a transmission link comprising a combination of a PDF and an IDF. In addition, the loss when splicing this IDF to a PDF should be kept as low as possible. It would also be desirable to provide such an IDF that is suitable for compensating dispersion of a large effective area PDF, such as a super-large effective area PDF.
The invention is embodied in an optical communications system including one or more optical transmission devices, one or more optical receiving devices, and at least one optical fiber cable coupled therebetween that includes at least one positive dispersion optical fiber and corresponding inverse dispersion optical fiber. According to embodiments of the invention, the inverse dispersion fiber has negative dispersion and a negative dispersion slope around the wavelength 1550 nm. The inverse dispersion fiber includes a doped core region with an index of refraction n1, a cladding region with an index of refraction n2, and a trench region, a first barrier region and a second barrier region with indices of refraction n3, n4, and n5, respectively, formed between the doped core region and the cladding region.
Inverse dispersion fiber according to embodiments of the present invention preferably has a chromatic dispersion of approximately xe2x88x9244 picosecond/(nanometer-kilometer) and a relatively large effective mode-field area, Aeff, e.g., greater than approximately 30.0 m2, both at a wavelength of 1550 nanometers. The various regions of the inverse dispersion fiber are manufactured in such a way that the refractive index value ranges preferably are, e.g., 0.709% less than (n1xe2x88x92n2)/n2 less than 1%, xe2x88x920.358% less than (n3xe2x88x92n2)/n2 less than xe2x88x920.293%, 0.194% less than (n4xe2x88x92n2)/n2 less than 0.237%, and xe2x88x920.045% less than (n5xe2x88x92n2)/n2 less than xe2x88x920.037%. In accordance with the preferred embodiment of the present invention, manufacture of the optical fiber includes manufacture of a core region having a diameter of approximately 4.83 xcexcm, a trench region having a diameter of approximately 11.01 xcexcm, a first barrier region having a diameter of approximately xe2x88x9218.44 xcexcm, and a second barrier region having a diameter of approximately 21.97 xcexcm.
The refractive index of the core preferably is graded to follow a power law, with an exponent of xcex3, where 1 less than xcex3 less than 7. The core region is doped with, for example, germanium or other suitable material. The trench region, the first barrier region, the second barrier and the cladding region are doped with, for example, germanium and/or fluorine, and/or other suitable material(s). Inverse dispersion optical fiber according to embodiments of the invention provides improved compensation of positive dispersion optical fibers, including existing positive dispersion optical fibers.